Annual Report 2004 of the Rupelian/Chattian Boundary Stratotype Working Group

Investigations on the Lower-Upper Oligocene Transition are the subject of a paper, submitted to the Geological Society of American Bulletin, on “Integrated stratigraphy of the Oligocene pelagic sequence in the Umbria-Marche basin (Northeastern Apennines, Italy): A potential GSSP for the Rupelian/Chattian boundary” by Coccioni and others, and presently under revision after peer reviews.

Complete and continuous sequences of fossiliferous marine rocks covering the Oligocene Epoch and exposed on land are rare, making it difficult to define cause-and-effect relationships of palaeoenvironmental changes in an accurate and precise chronostratigraphic framework. One exception is represented by the pelagic succession of the Umbria-Marche Apennines (central Italy) where a complete and continuous sequence of marly limestones, calcareous marls, and marls allows a detailed bio-, magneto-, and chemostratigraphic study of this Epoch. In addition, this sequence contains several biotite-rich volcaniclastic beds which provide the means for an accurate and precise radiometric calibration of the Oligocene time scale. The OLIS Working Group, including 24 scientists from several Universities and Institutions (Italy, The Netherlands, USA, Canada, France and Belgium), approached a detailed interdisciplinary stratigraphic study of the Oligocene with particular emphasis on the reconstruction of the events that characterize the Rupelian/Chattian boundary. One of the main results of such integrated stratigraphic analyses of the most representative Oligocene sections of the Umbria-Marche region is the interpolated age of 27.3 Ma (with a geochronologic uncertainty of 0.3 m.y.), for the Rupelian/Chattian boundary, located in the middle of chron C9n.2n at meter level 199 in the Monte Cagnero section, and corresponding to the P21a/P21b foraminiferal subzonal boundary. This age is approximately one million years younger than that reported in recent chronostratigraphic time scale compilations.

A formal proposal designating the Monte Cagnero (MCA) section, out of the three sections studied, as GSSP for the Rupelian/Chattian boundary is in preparation.

Investigations have been undertaken by researchers of the Universities of Padua, Ferrara, Urbino, Utrecht and Milan, and by the numerous scientists of the OLIS Working Group coordinated by Coccioni (University of Urbino), respectively.

Report by Isabella Premoli Silva, Chairwoman.